Biological purification of town sewage water has been known for a long time, and over recent years the trend has been away from methods using sludges activated with free cultures towards methods using fixed cultures in order to reduce the size of the installations.
In fixed cultures on fixed beds, bacteria are fixed on natural inorganic materials (clay, sand, . . . ) with the water to be treated passing therethrough in a determined reactor volume, and with air being added if required for the development of the bacteria. Because of its high particulate matter content, the water to be treated must be subjected to filtering and to pretreatment in order to reduce clogging of distributor manifolds and of the filter mass. In addition, the excess biomass that develops in the bed must be removed periodically, thereby causing the biological reactor to be operated discontinuously.
To avoid fixed beds being operated sequentially, reactors have been developed in which the bed of support material is fluidized by the water to be treated. That makes it necessary to select a support material having very small grain size. In this case also, the absolute necessity of obtaining uniform distribution of the water fed to the bottom of the reactor means that thorough pretreatment must be used in order to avoid any risk of the manifolds clogging, since that would destabilize the particle bed.
In addition, the biomass in the reactor may become stratified, giving rise to a higher concentration of support material in the lower portion thereof. Since attrition increases with increasing concentration, the quantity of biofilm that develops on the support particles is limited, and the purifying action suffers. Conversely, the lower concentration towards the top of the bed causes biofilm to accumulate on the particles.
Such accumulation causes particles to be lost progressively since their characteristics of size and density are modified sufficiently by the growth of biofilm to enable them to be entrained by the water passing through the reactor. Various devices have been developed (FR-A-2 626 868) for regenerating and reinjecting the particles that escape, but they complicate the structure of the reactor.
Another problem arises, in particular when it is desired simultaneously to purify carbon-containing matter and matter containing ammoniacal nitrogen, which problem is related to the different reproduction times of the bacteria in question.
The "carbon" bacteria which are heterotrophic bacteria have a much higher rate of growth than the "nitrogen" bacteria which are autotrophic bacteria. The carbon bacteria thus colonize the support particles very quickly, thereby preventing the autotrophic bacteria from fixing thereon. The autotrophic bacteria are therefore eliminated progressively with the treated water, and nitrification (transformation of ammoniacal N to nitric N) no longer takes place. This looses the advantage of having cultures that are fixed, namely the ability to dissociate kinetic constraints (reaction time to transform impurities) from physiological constraints (age of the biomass, or the transit time thereof through the reactor, which time must be greater than the reproduction time of the biomass in order to ensure that biomass continues to be present).